Mycotretus Lacordaire, 1842 is a Neotropical genus of beetles, being the second most speciose genus within Erotylidae. The older descriptions of Mycotretus species are anecdotal and none include information on male and female genitalia, which can have great taxonomic value. Our objective in the present work is to redescribe Mycotretus chilensis Crotch, 1876 and Mycotretus trifasciatus Guérin, 1956 providing the first descriptions of their mouthparts, metendosternite, male and female terminalia, as well as new data on their geographic distribution and host fungi. Additionally, the lectotype of M. chilensis is designated and Mycotretus bicinctus Guérin, 1949 is proposed as a new junior synonym of M. chilensis. Both M. chilensis and M. trifasciatus have great intraspecific variation and are sympatric or syntopic in a few localities. We conclude that morphological differences of the penial flagellum and the metendosternite can be used to diagnose species and may also have phylogenetic signal for Mycotretus species. The present work is our first step in an ongoing taxonomic revision of the genus Mycotretus.